The present invention relates to a method and apparatus for reducing the level of noise generating by a combustion process. More specifically, the present invention relates to a reduced noise generating combustor in which the fuel is circumferentially swirled around the combustion zone.
In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors. Traditionally, such combustors include a cylindrical liner that encloses a primary combustion zone in which an approximately stoichiometric mixture of fuel and air is formed and burned in a diffusion type combustion process.
Traditionally, fuel is introduced into the primary combustion zone by a fuel nozzle that directs the fuel at an angle with respect to both the radial and axial directions--that is, the fuel is simultaneously directed axially downstream and radially outward--so as to create vortices that aid in mixing the fuel and air and stabilizing the flame. However, the fuel nozzle did not circumferentially swirl the fuel around the combustion zone. Thus, if the direction of the flow from the fuel nozzle were projected onto a plane perpendicular to the axis of the combustor, the angle at which the fuel was directed would be exactly radially outward (i.e., the angle at which the fuel is directed is 0.degree. from the radially outward direction).
in the traditional approach, combustion air is introduced into the combustion zone in two ways. A minor portion of the combustion air is introduced axially into the combustion zone by an annular passage that surrounds the fuel nozzle. Vanes are distributed around the outlet of this annular passage and serve to swirl the axially introduced portion of the combustion air. The swirl imparted to the axially introduced portion of the combustion air aids the mixing of the fuel and air and the stabilizing of the flame.
The major portion of the combustion air is introduced radially into the combustion zone. This is accomplished by a plurality of tubular member--sometimes referred to as "air scoops"--distributed around the combustor liner. The air scoops direct the air radially inward (i.e., the angle at which the air is directed is 0.degree. from the radially inward direction) so that, like the fuel, the radially introduced portion of the combustion air is not circumferentially swirled around the combustion zone.
Cooling air is also introduced into the combustor axially and radially. The majority of such cooling air is introduced radially into the combustor downstream of the primary combustion zone and serves to cool the hot gases prior to expansion in the turbine. A smaller amount of cooling air is introduced axially into the combustor through annular passages formed between adjoining sections of the combustor liner and serves to cool the liner walls.
Unfortunately, the amount of nitrogen oxides (NOx) generated by the combustion process in the traditional combustors is high. Consequently, attempts have been made to reduce NOx generation by dramatically reducing the fuel/air ratio and, therefore, the combustion temperature. Unfortunately, such lean burning can reduce the stability of the combustion process. Efforts have been made to offset this reduction in stability by directing the fuel into the combustion zone so as to impart a circumferential swirl component to the fuel.
Unfortunately, it has been found that swirling the fuel increases the noise generated by the combustion process. It is therefore desirable to provide a combustor having the benefits associated with the swirling of the fuel without an appreciable increase in the sound level generated by the combustion process.